The invention relates to an arrangement for establishing a radio test loop in a radio unit comprising several radio transmitters and several radio receivers, wherein each radio transmitter comprises a test signal output which is adjustable during an operation of testing the radio transmitter from a high-impedance state to a low-impedance state, for outputting a test signal derived from a signal transmitted by the radio transmitter to a frequency converting means for conversion to the receiving frequency before looping to the respective radio receiver.
The operation of the radio sections of a radio transceiver can be supervised and tested by establishing a so-called radio test loop, that is, by feeding back a test signal derived from a radio-frequency test signal transmitted by the radio transmitter to the radio receiver after converting it to the receiving frequency. Radio units comprising several radio transceiver pairs, such as the base stations of cellular mobile radio systems, usually employ a single frequency converter to which test signals from the different transmitters are applied in successive turns. A separate cable is provided between the test signal output of each transmitter and a special combiner unit which combines the test signal outputs to the single supply line of the common frequency converter. When the number of transceiver pairs in the radio unit is high, such as 16 or 32, the realization of the radio test loop by this conventional technique requires a significant number of printed circuit boards and cables and, as a result, a substantial amount of space.